1. Field of the Invention
The present invention relates to an optical fiber for guiding ultraviolet light and laser machining apparatus using them.
2. Related Background Art
The ordinary optical fibers is made of pure silica glass and the core region thereof is doped with germanium. Such optical fiber is not suitable for transmission of light in the wavelength region of 150 nm to 400 nm (ultraviolet light). This is because the ultraviolet light is absorbed by the dopant of germanium added in the silica glass in order to raise the refractive index of the core region.
There is thus a known technique of making the core region of pure silica glass and doping the cladding region with fluorine F to decrease the refractive index of this region, thereby forming a waveguide capable of transmitting the ultraviolet light. However, bend loss is easy to occur in the waveguide, because the index difference (relative index difference) cannot be set at a sufficiently large value between the core and the cladding. A strong ultraviolet beam like the excimer laser light produces defects in the core region to increase transmission loss.
Proposed in order to prevent the production of defects was the optical fiber using pure silica glass and having the hydroxyl-doped core region. Such optical fiber is under practical use for transmission of light in the wavelength range of 250 nm to 400 nm (ultraviolet light).
However, since the conventional optical fiber having the hydroxyl-doped core region was generally used for power transmission, the diameter of the core region was very large, e.g., 200 xcexcm. It was also difficult to focus the light emerging from the fiber into a small spot diameter of about several micrometers even with the help of a condenser lens, because propagation of light was multimode propagation. For this reason, such optical fiber was not suitable for micromachining, analysis necessitating high power irradiation at a microscopic region, and so on.
If the ultraviolet light as described above is transmitted in a single mode, the light emitted from the fiber will appear with a uniform wave front and can be focused into a small spot diameter with the help of an optical system such as a condenser lens or the like even if the fiber has some large MFD (mode field diameter). For this MFD, the Petermann I formula is known as represented by the formula below. In the formula, xcfx86(r) represents an electric field distribution in the radial direction.   MFD  =            2      xc3x97                                    ∫            0            ∞                    ⁢                                                    φ                2                            ⁢                              (                r                )                                      ⁢                          r              3                        ⁢                          ⅆ              r                                                            ∫            0            ∞                    ⁢                                                    φ                2                            ⁢                              (                r                )                                      ⁢            r            ⁢                          ⅆ              r                                          
In order to make the fiber in the single mode on the other hand, it is necessary to satisfy V less than 2.405 in the following characteristic equation under the weakly guiding condition.   V  =                    2        ⁢                  xe2x80x83                ⁢        π            λ        ⁢    a    ⁢                            n          co          2                -                  n          cl          2                    
In this equation, V represents a normalized frequency, xcex represents a wavelength, a represents a radius of the core, nco represents a refractive index of the core region, and ncl represents a refractive index of the cladding region.
For example, let us consider an optical fiber capable of transmitting a laser beam of an ArF excimer laser in the single mode. Supposing the wavelength of the ArF excimer laser is 193 nm (0.193 xcexcm) and the index difference between the core and the cladding is 1% (nco=1.4585 and ncl=1.4439), the condition for the single mode is that the core diameter (2a) is less than 0.72 xcexcm.
However, this core diameter is too small to feed a large power. In order to transmit a practically applicable power, the core diameter needs to be at least 2 to 3 xcexcm and desirably not less than 4 xcexcm. This is because even if the core diameter is large the light with the uniform wave front can be focused by optical means such as a lens or the like.
It is also possible to increase the core diameter by decreasing the relative index difference between the core and the cladding. However, the bend loss increases with decrease of the relative index difference to cause a problem in practical use. Therefore, no single mode fiber for ultraviolet light has been realized heretofore.
The present invention has been accomplished under such circumstances and an object of the invention is, therefore, to provide an optical fiber capable of transmitting the ultraviolet light in a mode field diameter of practical size.
In order to solve the above problem, an optical fiber according to the present invention is an optical fiber in which a core region and a cladding region surrounding the core region are made of a base material comprising silica glass as a principal component and in which a plurality of holes extending along an axial direction are arranged at least in the cladding region, wherein the base material has a hydroxyl mole fraction of not less than 10 ppm, or a fluorine content of not less than 0.2% by weight.
Another optical fiber according to the present invention is an optical fiber in which a core region of a hole is surrounded by a base material comprising silica glass as a principal component to form a cladding region and in which a plurality of holes extending along an axial direction are arranged in the cladding region, wherein the base material has a hydroxyl mole fraction of not less than 10 ppm, or a fluorine content of not less than 0.2% by weight.
Ultraviolet resistance of silica glass can be enhanced by adding hydroxyl or fluorine thereto. Addition of fluorine presents the effect of shifting the absorption edge to the shorter wavelength side to enhance ultraviolet transmission characteristics further. Since the placement of the holes in the cladding region can greatly vary the average refractive index of the cladding region, it becomes feasible to increase the relative index difference, as compared with the conventional fibers, and it thus becomes easy to produce the optical fiber having a small bend loss and a large MFD while transmitting the ultraviolet region.
It is preferable to employ a configuration wherein these holes are arranged in a hexagonal lattice array in a cross section orthogonal to the axial direction and wherein a relation of d/xcex9xe2x89xa60.2 or xcex9xe2x89xa62.35d/xcex9+0.5 holds between a diameter d of the holes and a pitch xcex9 between adjacent holes. Instead, it is also preferable to employ another configuration wherein these holes are arranged in a square lattice array in a cross section orthogonal to the axial direction and wherein a relation of xcex9xe2x89xa6xe2x88x9210(d/xcex9)2+10(d/xcex9)xe2x88x921.1 holds between a diameter d of the holes and a pitch xcex9 between adjacent holes. These holes may be arranged on circumferences of concentric circles centered about the fiber axis in the cross section orthogonal to the axial direction.
The settings as described above permit the MFD to be determined as a practical size, facilitate the production of optical fiber, and permit the optical fiber to be produced with stable quality.
The base material preferably contains hydrogen of not less than 1016 molecules/cm3. This makes it feasible to suppress the production of defects in the core region due to the strong ultraviolet light and thereby maintain transparency to the ultraviolet light over a long period of time.
A laser machining apparatus according to the present invention is constructed by combining either of the optical fibers according to the present invention, as described above, with a condenser lens placed at an exit end of the optical fiber. Since the above-stated optical fibers according to the present invention can have a large numerical aperture, it is feasible to reduce the size of the condenser lens, design the apparatus in short path lengths, and thereby decrease the size of the machining apparatus.